Various types of projection television systems have been developed and commercialized in recent years. Although such projection television systems provide a large viewing screen, they tend to be more expensive than standard CRT based systems, are more cumbersome and, most importantly, they tend to provide a relatively dim image on the viewing screen. The image produced, as well as the expense and size of a projection television system, is often directly dependent on the optical system used in such systems.
An example of an optical system in a prior art system is described in U.S. Pat. No. 5,084,807, granted to Thomas S. McKechnie on Jan. 28, 1992. The prior art system described in U.S. Pat. No. 5,084,807, is illustrated in FIG. 1 of the patent, and describes an arrangement in which light from three tungston halogen lamps is transmitted and/or reflected through a crossed pair of dichroic mirrors and a dichroic prism to pass through three Liquid Crystal Displays (LCD's). The three LCD's modulate three channels to create the red, green and blue portions of a television picture.
The crossed dichroic mirrors break the white light source into red, blue and green portions of the spectrum. These three color portions are then recombined by way of the dichroic prism and the recombined light is projected by way of a projection lens to a projection screen.
Although this prior system has many advantages, the use of a first set of crossed dichroic mirrors and a second dichroic mirror prism adds to the complexity and size of the device and also, of course, to the expense of the completed projection television system.
Another prior art system is described in U.S. Pat. No. 4,687,301, granted to Ledebuhr on Aug. 18, 1987. This patent describes a plumicon prism based LCD projector. With this type of system, a substantial amount of glass is used which makes the prism arrangement large and very expensive which is, of course, a disadvantage in a projection television system.
Another prior art LCD projection system is described in U.S. Pat. No. 5,012,274, granted to E. Dolgoff on Apr. 30, 1991. The system described in this patent utilizes three LCD's, one displaying red, one green and one blue, picture data. Each of the LCD's is illuminated with light of the appropriate color, i.e., a red light source, a blue light source, and a green light source. Light from the red, green and blue light sources, is collected by a condenser, collimated by collimating optics, and passed through a series of dichroic mirrors to form a full color image on a screen.
Although the system described in this patent also has been widely used, there are certain disadvantages. More particularly, the back focus of the projection lens used is much larger than what would be found, for example, in other prism designs. This makes the design and production of such a system very expensive. In essence, the long back focus of the projection lens leads to problems in producing a commercial and reliable projection television system.
It is, therefore, an object of this invention to provide a beam combiner for an LCD projector, which is useful for both front and rear projection, but has a compact light path and is simple to manufacture.
It is a further object of the instant invention to produce an LCD color projection television system that has a cost effective dichroic mirror system.
It is a still further object of the instant invention to provide a dichroic combiner for a projection television system that is simple, inexpensive, easy to manufacture, and which will result in a cabinet size for a commercial projection system that is acceptable to the consumer.
Another prior art optical system, remote from the projection television field, is described in the Journal of the International Society of Optical Engineering, September 1994, Volume 33, Number 9. This system does not relate to projection television systems, but describes a single axis, three color light source system suitable for high speed photography.
The system described is an optical arrangement which consists of three white light sources, a number of collimating lenses, and three dichroic filters mounted in a penta holder. Light from a first source passes through the first collimating lens producing a parallel beam of light. This light is then passed through a dichroic filter, which is made to transmit red light only, while reflecting blue and green light. The transmitted red light reflects off the inner surface of a second dichroic filter which transmits only green light reflecting red and blue. Finally, the red beam reflects off a third filter, which is manufactured to transmit only blue light. The combined red, green and blue light sources pass out of the penta arrangement, where it is focused with a collimating lens.
This system, requires the dichroic filters to be specially manufactured as their performance is very sensitive to the light incident angle. In addition, the penta holder described in this reference must be very accurately machined from a stainless steel block and the critical components and light sources are mounted on rods screwed into the block. Accordingly, although this system presents a optical arrangement which may be of interest in a projection television system, it would obviously be very expensive and require special manufacturing techniques, all of which would be unsuitable for use in a commercial projection television system.